According to the documentation of JXCollapsiblePane.setContentPane(Container contentPanel), the default content pane implements Scrollable and returns true from Scrollable.getScrollableTracksViewportHeight() and Scrollable.getScrollableTracksViewportWidth().

Indeed, the default content pane is of type JXPanel which satisfies the above requirements. Yet, the collapsible pane is not scrollable.

To reproduce the problem, run the example below and try to minimize the window size. The scroll bars do not appear, and the content pane is not scrollable.